The invention relates to an evaporator boat for an apparatus for coating substrates in a vacuum coating chamber, consisting of a shallow, trough-like piece of an electrically conductive material, the boat being heated by direct passage of electrical current through it.
Evaporator boats are known--especially series evaporators--which are rectangular in plan and are provided at their two vertical short sides with clamping surfaces, or flats, by which they are held on the boat holder.
DE-A 40 27 034 discloses apparatus for the continuous coating of substrates in band form, in a vacuum coating chamber having a plurality of evaporator boats of approximately equal size and configuration forming a bank of evaporators arranged at approximately equal distances apart, parallel to one another and lengthwise of the direction of band movement. The boats are all formed from an electrically conductive ceramic and heated by direct passage of electric current through them. The apparatus has a device for the continuous delivery of wire to the evaporator boats for evaporation, and the individual evaporator boats are arranged each offset from the other, all the evaporator boats together covering a narrow coating zone extending transversely from the direction of band movement.
The offset arrangement is said to solve the problem of reducing the interaction of the individual sources and thereby to improve the coating uniformity. The individual evaporator boats themselves have a rectangular cross section, and the recesses for containing the molten material all have a rectangular plan.
DE-A 970 246 discloses apparatus for the continuous evaporation of endless material wherein a plurality of directly heated evaporators lie one behind the other in a row. Here, the attempt has been made to overcome a disadvantage of the known type of evaporator of rectangular bottom surface in the recess. The disadvantage is due to the fact that the evaporating material, being an excellent conductor for the heat inducing electrical current, partially short-circuits the evaporator, so that only those portions of the evaporator that are present between the individual recesses serve for heat generation. This condition results in a non-uniform temperature distribution in the boat and non-uniform coating of the finished product. According to this earlier disclosure, this disadvantage is said to be eliminated by the fact that the evaporator has thinner cross section between the individual recesses so that the electrical resistance of the evaporator is approximately constant and independent of the amount of the evaporating material in the chambers.
DE-A 40 16 225 discloses a serial evaporator for vacuum coating apparatus, especially band coating apparatus, is described, consisting of a plurality of evaporators of individually controllable power, heated by passing electric current through them, and lying against pin-like electrical feeders. These pin-like feeders are held by an electrically conductive support extending over the entire length of the serial evaporator, the feeders of the one polarity being connected electrically to the support and the feeders of the other polarity being brought in an insulated manner through the support and connected to insulated conductor wires. The evaporators rest on bearing means configured preferably as cylinder sections at the upper ends or terminals of the feeders. For this purpose the bearing means are made from a current-conducting material, such as ceramic.
This older serial evaporator solves the problem of holding the evaporators in pairs between the electrical feeders and of good electrical contact during the vapor depositing process. But even with this pair-wise arrangement of evaporators, the problem of optimal temperature distribution within the boat contents to help obtain a uniform coating is not yet fully solved.